Dropdown menu fails after adding mytheme

After I added ‘mytheme’ my single dropdown menu does not allow the reader to pick anything other than the first item. When the mouse is below the first item the menu disappears. My theme has only a little CSS and a revised modular text twig. All my pages (home and the dropdown with two items) have only a title and a line of content.
I cannot find anything about dropdown in the manual or in a config file.
Totally baffled. Does anyone know what I am missing?

There are a few themes that dont work with the multi drop down option. I’ve noticed bootstrap4 works with it, if you’d like to see what they do different.